Electric motor-driven equipment including mobile electronic equipment such as mobile computers and digital cameras are required to be downsized and lightweight and the market of electric motor-driven equipment has been rapidly growing in recent years. As a power supply of such electric motor-driven equipment, sealed nickel metal-hydride batteries provide energy per unit volume and unit weight higher than nickel cadmium storage batteries and lead storage batteries and are excellent in terms of resistance to over-charge and over-discharge so that they are popularly being used as environment-friendly clean power sources. Additionally, sealed nickel metal-hydride batteries are finding applications in the field of power sources of hybrid electric vehicles (HEVs), electric motor-driven tools and electric toys that have been driven heretofore by nickel cadmium batteries and require high output power performance and a long service life.
Nickel metal-hydride batteries to be applied to power sources of HEVs, electric motor-driven tools and electric toys having a heavy load are required to be improved in terms of output power performance particularly at a low temperature (e.g., 0° C.) at least without reducing the charge/discharge cycle performance. Nickel metal-hydride batteries desirably show an output density not less than 400 W/kg, preferably not less than 600 W/kg, at a low temperature (0° C.) for applications having a heavy load such as power sources of HEVs and electric motor-driven tools. Additionally, the nickel metal-hydride battery installed in a HEV at a position where the temperature of the ambient air can be raised desirably has a cycle life of not less than 400 cycles, preferably not less than 500 cycles, at a high temperature (e.g., 45° C.).
The output power performance of a nickel metal-hydride battery can vary mainly depending on the discharge ability of the hydrogen absorbing electrode thereof. Proposals for activating hydrogen absorbing alloy powder by immersing the hydrogen absorbing alloy powder in a weakly acidic aqueous solution or a hot alkaline aqueous solution have been made to date to improve the high-rate discharge ability of a hydrogen absorbing electrode. For example, methods of surface-treating hydrogen absorbing alloy powder by means of a weakly acidic aqueous solution with a pH value between 0.5 and 5 have been proposed. (e.g., refer to Patent Document 1.)
Patent Document 1: JP-A-07-73878 (page 3, paragraph 0011)
Additionally, methods of immersing hydrogen absorbing alloy powder in an aqueous sodium hydroxide solution with a sodium hydroxide concentration of 30 to 80 wt % at a temperature not lower than 90° C. have been disclosed. (e.g., refer to Patent Document 2.)
Patent Document 2: JP-A-2002-256301 (page 3, paragraph 0009)
According to Patent Documents 1 and 2, the coats of oxide or hydroxide formed on the surfaces of particles of hydrogen absorbing alloy powder are removed to recover clean surfaces and additionally, the hydrogen absorbing alloy powder is activated because a layer containing Ni as main component is formed on the surfaces. Thus, the proposed methods provide an advantage of curtailing the formation process that is executed for the purpose of activation and improving the high-rate discharge ability of the hydrogen absorbing electrode. However, Patent Document 1 only shows the discharge capacity (mAh) while discharging at a rate of 1 ItA at 0° C. (the discharge rate being lower than the discharge rate in the evaluation of the output power performance as will be described hereinafter) as discharge ability at low temperature and does not show any output power performance as determined according to the output power performance of the present invention (the output power performance (W) as determined from the voltage at the tenth second (10 seconds after the start of discharge) as will be described hereinafter). Patent Document 2 only shows the discharge capacity (the ratio relative to the discharge capacity while a discharging at 25° C.) when the discharge takes place with an electric current equivalent to 4 ItA at −10° C. as discharge cut voltage of 0.6 V (which is lower than the discharge cut voltage of 0.8 V according to the present invention as will be described hereinafter) and does not show any output power performance. In short, neither Patent Document 1 nor Patent Document 2 mentions output power performance. Additionally, neither Patent Document 1 nor Patent Document 2 mentions the amount of hydrogen absorbing alloy powder per unit area of a hydrogen absorbing electrode. Therefore, there is a great possibility that neither the method of Patent Document 1 nor that of Patent Document 2 can remarkably improve the output power performance at a low temperature.
Furthermore, the methods of Patent Documents 1 and 2 are accompanied by a disadvantage that, when the charge/discharge cycle is repeated at a high temperature (e.g., 45° C.), the corrosive reaction of hydrogen absorbing alloy powder can be accelerated and oxygen is apt to be generated at the nickel electrode during the charge time if compared with the room temperature to further promote the corrosion of hydrogen absorbing alloy powder so that it is difficult to maintain the cycle performance by either of the methods of Patent Documents 1 and 2.
Hydrogen absorbing electrodes with an improved corrosion resistance of hydrogen absorbing alloy powder without reducing the output power performance obtained by adding a yttrium (Y) compound or a compound of a light rare earth element selected from lanthanum (La), cerium (Ce) and praseodymium (Pr) to the hydrogen absorbing alloy powder of the hydrogen absorbing electrode have also been proposed. (e.g., refer to Patent Document 3.) Hydrogen absorbing electrodes with an effect of inhibiting corrosion of a hydrogen absorbing alloy and an improved durability obtained by making hydrogen absorbing alloy powder immersed in advance with an alkaline aqueous solution or a weakly acidic aqueous solution contain the element of holmium (Ho), erbium (Er), ytterbium (Yb) or thulium (Tm) or a compound of any of them have also been proposed. (e.g., refer to Patent Document 4.)
Patent Document 3: JP-A-11-260361
Patent Document 4: JP-A-09-7588
The additives described in Patent Documents 3 and 4 have an excellent anti-corrosion effect against corrosion of hydrogen absorbing alloy powder. Particularly, corrosion of hydrogen absorbing alloy powder is inhibited to remarkably improve the cycle performance when powder of hydroxide or oxide of Er or Yb is added. However, Patent Document 3 does not mention anything about output power performance and hence the object of the invention of Patent Document 3 is not to improve the output power performance. While Patent Document 4 mentions improvement of output power performance, the invention of Patent Document 4 is intended not to positively improve the output density than ever but to inhibit the reduction of the output density that inevitably arises by inhibiting the phenomenon that the surfaces of particles of hydrogen absorbing alloy powder are covered by a high resistance coat because, when a Y compound is added to hydrogen absorbing alloy powder, the surfaces of particles of hydrogen absorbing alloy powder are inevitably covered by a high resistance coat. The method described in Patent Document 4 can hardly reduce the reaction resistance of a hydrogen absorbing electrode at a low temperature and is not effective for improving the output power performance at a low temperature.
Since the high-rate discharge ability and the charge acceptability of the hydrogen absorbing electrode (negative electrode) is inferior relative to the nickel electrode (positive electrode), the capacity of the negative electrode needs to be sufficiently large relative to the capacity of the positive sufficiently large relative to the capacity of the positive electrode in order to secure a sufficient charge reserve and a sufficient discharge reserve. It is a general practice to select a value between 1.5 and 1.7 for the capacity ratio of the negative electrode capacity to the positive electrode capacity (N/P ratio). The rate of filling an active material per unit area of the hydrogen absorbing electrode of a nickel metal-hydride battery is conventionally set to be between 0.16 and 0.20 g/cm2 in order to confine the N/P ratio to the above range and secure a high capacity.
However, it has been found that, when a large value is selected for the rate of filling an active material in the hydrogen absorbing electrode as in the case of conventional batteries, the reaction resistance of the hydrogen absorbing electrode is high particularly at low temperatures and the target output power performance can hardly be achieved. A conceivable measure to overcome the above problem may be to change the composition of the electrolyte. However, if the output power performance is improved at a low temperature by changing the composition of the electrolyte, other performances including the cycle performance can become degraded.
Alkali batteries having a negative electrode made of a hydrogen absorbing alloy and having a small capacity per unit area (a capacity per unit area of 10 to 40 mAh/cm2) have been proposed. Such an alkali battery can reduce the electric resistance of the negative electrode and improve the discharge ratio for discharges at low temperatures (capacity measured when discharged at a low temperature/battery capacity) when the above capacity value is selected. (e.g., refer to Patent Document 5.)
Patent Document 5: JP-A-11-86898
However, the discharge ratio shown in Patent Document 5 is the value observed while a discharging at 1 CmA (1 ItA) and at 0° C. The discharge rate cited in Patent Document 5 is low if compared with the discharge rate in the evaluation of the output power performance as will be described hereinafter and the Patent Document 5 does not show any output power performance either. The hydrogen absorbing electrode described in Patent Document 5 shows a large reaction resistance because the hydrogen absorbing alloy of the hydrogen absorbing electrode is not highly active as active material and hence the effect thereof on the improvement of performance is small when a high-rate discharge is realized at a low temperature while its charge acceptability is poor in the initial cycles to generate hydrogen to a large extent during a charge and the cycle performance can be degraded probably because the oxygen absorbability is poor and hence the electrolyte is consumed to a large extent.
In a conventional cylindrical nickel metal-hydride battery as shown in FIG. 4, the sealing plate 0 of a lid (the lid having a knob-like cap 6, the sealing plate 0 and a valve 7 arranged in the space defined by the cap 6 and the sealing plate 0, a gasket 5 being mounted at the peripheral edge of the sealing plate 0, the lid being clamped along the peripheral edge thereof by folding the open end of a cylindrical container 4 with a bottom so that the lid and the container are held in airtight contact with each other by way of the gasket 5) that operates as one of the terminals (positive electrode terminal) and the upper current collecting plate (positive electrode current collecting plate) 2 fitted to the upper ends of a rolled electrode assembly are connected to each other by a ribbon-like current collecting lead 12 as shown in FIG. other by a ribbon-like current collecting lead 12 as shown in FIG. 5. For the conventional battery, the current collecting lead needs to be made flexible in order to mount the lid on the open end of the container 4 after welding the ribbon-like current collecting lead 12 and the inner surface of the sealing plate 0 and the current collecting lead 12 and the upper current collecting plate 2. For this reason, the current collecting lead connecting the welded point of the current collecting lead 12 and the inner surface of the sealing plate 0 and the welded point of the current collecting lead 12 and the upper current collecting plate 2 is normally six to seven times longer than the gap between the sealing plate 0 and the upper current collecting plate 2. Thus, the current collecting lead itself has a large electric resistance because of such a length of the current collecting lead. This may be one of the reasons of the low output power performance of the battery.
As described above, there have been attempts to enhance the activeness of hydrogen absorbing alloy powder as active material by surface reforming of hydrogen absorbing alloy powder. However, a satisfactory high output cannot be achieved particularly at low temperatures simply by such an attempt and other performances such as the cycle performance can be degraded by the attempt to enhance the output power performance.